When is a group of stars not a galaxy?

Are there impostors lurking among the many millions of galaxies identified so far? No one can give a clear answer because there is as yet no formal definition of what a galaxy is. But a pair of astronomers are now putting the question of what defines a galaxy to a public vote, in the hope of reaching a consensus and avoiding the sort of controversy that surrounded Pluto being stripped of its status as a planet.

While a typical galaxy contains billions of stars, a number of tiny galaxies have been found in recent years that do not fit the classic picture and instead resemble the groups of stars known as star clusters. So which are they?

"There is no simple definition of what can be a galaxy or a star cluster," says Pavel Kroupa of the University of Bonn in Germany. "Where does one draw the line?"

Online survey

To try and settle the matter, Kroupa and Duncan Forbes of Swinburne University of Technology in Hawthorn, Victoria, Australia, have launched an online survey to poll astronomers on what they think the defining features of a galaxy should be.

Star clusters and galaxies both contain stars bound together by gravity, but while the members of a star cluster are thought to form simultaneously from a collapsing ball of gas, galaxies have richer histories. In the most popular cosmological model, they form along swathes of dark matter and contain enough gas to form many generations of stars.

Yet the distinction is not always clear-cut. Take Omega Centauri (pictured), a round swarm of stars that orbits the Milky Way and is visible to the naked eye. It has long been classified as a star cluster, but there is now evidence that it contains multiple generations of stars, suggesting it is actually the remnant of a galaxy.

Five criteria

In a paper to appear in Publications of the Astronomical Society of Australia, Forbes and Kroupa have offered five possible criteria for determining whether an object is a galaxy: the presence of dark matter, multiple generations of stars, satellite star clusters, a minimum size, and the time it takes for gravitational interactions between stars to slow them all down to roughly the same speed. In galaxies, stars tend to be farther apart, so this smoothing-out process takes longer.

Response to the poll has been mixed. Some say dark matter should be the only defining factor, but this raises practical problems. Looking for dark matter by searching for slight deviations in star motions is tricky in low-mass objects, says Manoj Kaplinghat of the University of California, Irvine. Citing the case of a dim object called Willman 1, he says: "If we could unambiguously say that it is held together by dark matter, nobody would debate if it is a galaxy."

Others say there may never be an unambiguous dividing line. "We don't understand well enough how galaxies and star clusters form to know if any clear division between them actually exists," says Michael Drinkwater of the University of Queensland in Brisbane, Australia. "There may not be any real dividing line out there to find."

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